1. Field of the Invention
The present invention relates to a variable speed tool, and more particularly to a tool, such as an electric drill, a screw driver, and a hammer drill, which has a multistage variable speed transmitting mechanism for outputting various rotational speeds as being driven by an electric motor, a pneumatic motor, or a hydraulic motor and a variable speed control method for controlling the variable speed tool.
2. Description of the Prior Art
A variable speed tool, such as an electric drill, a screw driver, and a hammer drill, has a multistage variable speed transmitting mechanism that may output various rotating speeds according to a variation of the load torque. The multistage variable speed transmitting mechanism of the variable speed tool typically comprises a multistage transmitting gear train and a gearshift device, which are coupled between a motor and an outputting shaft. An axial movement of the gearshift device changes the relationship of engagement with the multistage transmitting gear train, thereby varying transmission ratio and outputting various rotating speeds. In the state of the art, the operation of adjusting the gearshift device is performed manually. For example, U.S. Pat. No. 6,655,470 discloses a manual speed changing mechanism. The speed changing mechanism comprises a movable gear that is connected via an engaging member to a push button that is mounted to a base and axially movable. An operator pushes the push button for axial movement to cause the movable gear to move axially so that the movable gear selectively locks a first inner teeth ring or a second inner teeth ring to the base, or the movable gear simultaneously engage both the first and second inner teeth rings to have them rotating together, whereby three different outputs different rotational speeds and different torques can be provided. However, such a manual operation requires the operator to constantly watch the operation condition and variation of load of the tool for determining the timing for adjusting the speed changing mechanism. Such a manual operation is not friendly to the operator.